The clinical study by ultrasonic echography of blood flows is actually realized with apparatus which utilize the Doppler effect which takes into account the frequency difference, denoted Doppler frequency f.sub.D, between the wave transmitted by a piezoelectric transducer and the wave received after interaction with the flow considered. The speed V of the blood flow is linked to f.sub.D by the relation: EQU f.sub.D =2(V/C)f.sub.E Cos.theta.
f.sub.E being the frequency of the transmitted ultrasound wave, .theta. being the angle between the ultrasound beam and the direction of the flow, and C the speed at which the sound travels.
The known Doppler devices render it more particularly possible for medical practitioners to have to interesting parameters, such as speed spectograms which show the distribution of the speed of flow V as a function of the time t. Nevertheless, the result obtained has a certain inaccuracy which is inherently linked to the basic principle of the method and which results from the fact that the frequency f.sub.E transmitted by the piezoelectric transducer has a certain distribution which has repercussions on the speed V via the above-mentioned relation (1).
Therefore, the technical problem to be solved by the present invention is to provide a device for measuring and displaying physiological parameters of a blood flow, as defined in the opening paragraph, which renders it possible to obtain, in addition to a more precise speed spectogram, the determination of additional physiological parameters with the desired precision, which so far have been unaccessible.
Amongst the devices capable of giving a speed measurement independently of the ultrasound frequency, there are those devices which function on the temporal correlation principle described in European Patent Application No. 0 225 667, and whose unit for measuring the flow speed includes an intercorrelation circuit which from two consecutive echoes supplies values of the correlation function, and a multiplexing-interpolating circuit which from the values of the correlation functions produce an estimate of the speed V(t,z).
The precision obtained in the measurement of the flow speed renders it possible to provide, in addition to the display of the blood flow in mode M, for example, and the representation of the speed spectogram, the calculation of further physiological parameters which so far have been inaccessible by the conventional Doppler system because of their poor resolution at deeper levels.